1. Field of the Invention
The present invention relates to an apparatus and method for detecting a code of a direct sequence spread spectrum signal, and more particularly, to a direct sequence spread spectrum signal code detecting apparatus that can reduce influence of noise and data when a pseudo noise (PN) code of a direct sequence spread spectrum signal, which does not have information on a PN code, is detected by using a triple correlation function in a Code Division Multiple Access (CDMA) communication system, and a method thereof.
2. Description of the Related Art
A Code Division Multiple Access (CDMA) communication system, which was adopted as a main wireless mobile communication method in Korea, is based on a frequency band spread communication technology. Due to its property that it is hard to be wiretapped and strong against interference radio wave, the frequency band spread communication technology has been used in military devices. According to the frequency band spread communication technology, data to be transmitted in a transmitter are transmitted by coding the data by using a predetermined code to thereby spread the spectrum of the data and generate a spread spectrum signal, and transmitting the spread spectrum signal. A receiver receives the data by dispreading the received spread spectrum signal by using the code to thereby recover the data.
Generally, a receiver includes PN code information of transmitted spread spectrum signal in a commercial CDMA communication system, it receives desired data by dispreading the received signal with the PN code. However, a military communication system or a spectrum surveillance system should acquire data from received spread spectrum signal by detecting a PN code from the received spread spectrum signal without information on what PN code is used in the received spread spectrum signal under the assumption that the received spread spectrum signal exists within a broad bandwidth of the receiver.
Despite the Low Probability of Interception (LPI) characteristic of spread spectrum signals, researchers have studied to detect a PN code to extract data from the spread spectrum signals without PN code information.
Conventional PN code detection methods include a method using an interception receiver or a radiometer receiver, a Gardner's method based on cyclostationarity, and a higher order statistical signal processing method.
The conventional method using an interception receiver or a radiometer receiver has a shortcoming that the PN code detection performance is poor.
The method suggested by Gardner is based on cyclostationarity that a code is repeated in spread spectrum signal. The Gardner's method shows better detection performance than the method using an interception receiver or a radiometer receiver. However, it can only confirm the presence of a spread spectrum signal and it cannot exactly extract a PN code from a received spread spectrum signal.
On the contrary, the high-degree statistical signal processing method has an advantage that it can check the presence of a spread spectrum signal and extract a PN code form a spread spectrum signal. A representative form of the high-degree statistical signal processing method is to use a triple correlation function (TCF).
Hereinafter, a conventional method of extracting a PN code by using a triple correlation function will be described.
The following Equation 1 is a triple correlation function.
                                          R                          m              i                                ⁡                      (                                          τ                1                            ,                              τ                2                                      )                          =                              1            N                    ⁢                                    ∑                              n                =                1                            N                        ⁢                                                            r                                      m                    i                                                  ⁡                                  (                  n                  )                                            ⁢                                                r                                      m                    i                                                  ⁡                                  (                                      n                    +                                          τ                      1                                                        )                                            ⁢                                                r                                      m                    i                                                  ⁡                                  (                                      n                    +                                          τ                      2                                                        )                                                                                        Eq        .                                  ⁢        1            
where mi denotes an arbitrary PN code; rmi(n) denotes a sampled discrete signal of a spread spectrum signal spread by mi; τ1 and τ2 denote discrete time delay; and N denotes the number of discrete samples used for correlation.
A triple correlation function value obtained based on the triple correlation function value presented in the Equation 1 has a value of 1 or −1/N with respect to all τ1 and τ2. Herein, 1 is a peak value of the triple correlation function value for a PN code. When triple correlation function is operated with respect to an arbitrary discrete sequence, the triple correlation function of a PN code does not produce any peak value pattern. Thus, it is possible to check whether a spread spectrum signal exists within a given bandwidth by comparing a calculation result with an ideal peak value pattern of the triple correlation function of a received discrete signal.
After it is checked whether there is a spread spectrum signal by using the triple correlation function value, the length of the PN code is detected. The triple correlation function value has a pattern repeated at a predetermined interval, when the number N of discrete samples used for correlation is sufficiently large. The period of the repeating pattern becomes the length of the PN code.
Subsequently, when there is a peak value of the triple correlation function value, the PN code can be detected by using a discrete time delay sequence pair (τ1, τ2). First, the discrete time delay sequence pair (τ1, τ2) is acquired, when there is a peak value of the triple correlation function value. The pair is called a peak location, and a set of peak locations can be expressed as the following Equation 2.Pmi={(τ1,τ2)|Rmi(τ1,τ2)=1}  Eq. 2
The set Pmi is determined based on an arbitrary PN code mi. Since the set is predetermined differently according to each PN code, it is possible to determine a PN code.
As described above, the conventional PN code detection method can be used to detect a PN code in a spread spectrum signal without PN code information. However, there is a problem that reliability is poor because the triple correlation function value is under a strong influence of noise and data spread by a PN code.